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She has a serious health condition is how I would say it. Illness automatically sounds like it is probably somewhat transient. Health condition sounds more long term, like diabetes.
– aparente001Sep 30 '16 at 5:27

@aparente001 - There's no indication that the condition is "serious". It could be just a minor annoyance.
– Hot LicksOct 1 '16 at 2:32

@HotLicks - Oh. Well, in that case, long term illness.
– aparente001Oct 1 '16 at 2:39

@TheBitByte ... Yes, you are wrong. "Chronic" means it lasts a long time. It need not be high level at all.
– GEdgarSep 29 '16 at 21:18

1

Just a comment for people not familiar to medical descriptions of diseases. There are three general terms used for describing the behaviour of diseases: acute (the disease appearing suddenly and quickly), chronic (the disease lasts a very long time, typically the lifespan of the person) and severe (the disease affects the person very extremely). Some diseases may have several of these behaviours - SARS for example is both a Severe and Acute form of Respiratory disease.
– slebetmanSep 30 '16 at 0:33

Historically people have either been acutely ill (an immediate but potentially curable problem e.g. kidney stones) or chronically ill, the latter meaning something that people can live with but where the symptoms cannot be substantially relieved (e.g MS or Parkinson's disease)

Recently I have heard a lot of mention of life-changing injuries. I am not sure if that term can be applied to illnesses as well.

Let me suggest "incurable", since "chronic" (which is what first came to mind) does not speak to whether it can be cured or not, merely that it is steady-state. Clearly you don't like "incurable" since you asked for a replacement, but it may be the closest to the meaning you want.

Depending on the actual nature of the illness, another possibility is congenital, which means a condition that is present from birth, whether due to environmental or genetic causes.

While not not all congenital conditions are incurable, some are, and if you are talking about an adult when you say they have a congenital condition, it strongly implies that it it is difficult or impossible to cure, because they've had it since infancy and still have it as an adult.

Congenital also can mean "inherent in one's nature" which implies that one is stuck with the condition and can't change it, as when someone is called a "congenital liar".

"Congenital" implies, as you suggest, a condition present from birth. In some cases the condition can be corrected through surgery, and in some other cases the condition may never "surface" as an overt disease. And modern medicine, in a few cases, can actually use genetic engineering to "cure" the condition.
– Hot LicksSep 30 '16 at 2:09